This invention relates to an improved method of manufacturing Rxe2x80x94Fexe2x80x94B bonded magnets, and more particularly to an improved method of manufacturing highly corrosion-resistant Rxe2x80x94Fexe2x80x94B bonded magnets exhibiting outstanding corrosion resistance and bonding characteristics wherein, using dry barrel polishing, polishing material powder and bonded magnet grindings, or those together With inorganic powder, are imbedded and sealed in the pores of the magnet, and modification is effected in a surface smoothing treatment, after which a non-electrolytic plating layer is formed directly to the surface of the magnet material, a uniform electrically conducting layer is formed as an underlayer, and a highly corrosion-resistant electrolytic plating layer is deployed that can be efficiently formed with good volume productivity, without limiting the plating solution for electrolytic nickel plating or the like.
Today, in the rubber magnets and plastic magnets called bonded magnets, which are made in various shapes such as ring shapes and disk shapes, advances are being made toward higher performance, moving from conventional isotropic bonded magnets to anisotropic bonded magnets, and from ferrite-based bonded magnets to rare earth bonded magnets which exhibit higher magnetic properties, and also from Smxe2x80x94Co magnetic materials to Rxe2x80x94Fexe2x80x94B bonded magnets which use Rxe2x80x94Fexe2x80x94B magnetic materials exhibiting, in sintered magnets, high magnetic properties, with a maximum energy product of 50 MGOe or higher.
There is a problem with Rxe2x80x94Fexe2x80x94B magnets in that they rust easily due to their composition which contains large quantities of Fe and a component phase that oxidizes extremely readily, and the surfaces thereof have been coated with resin layers of various compositions by electrodeposition, spraying, immersion, or impregnation, etc. (cf. Japanese Patent Application Laid-Open No. H1-166519/1989, Japanese Patent Application Laid-Open No. H1-245504/1989).
With the resin coating methods used to date for enhancing the corrosion resistance of Rxe2x80x94Fexe2x80x94B bonded magnets, as in the case of ring-shaped bonded magnets using a spraying method, for example, coating material loss is great, and many process steps are involved due to the necessity of reversing the front and back, and there has also been the problem of deterioration in film thickness uniformity.
With the electrodeposition method, moreover, although the film thickness is uniform, each magnet must be attached to an electrode, which requires more process steps and is unsuitable for small magnets. In addition, the electrodes leave marks that must be removed after the coating is made, thus requiring a touch-up operation. Hence this method is problematic in that it requires many process steps and is unsuitable for small magnets. Using the immersion method, it is very difficult to obtain coated films of certain uniform thickness due to dripping and other problems. With porous bonded magnets, moreover, the pores are not adequately filled in, resulting in such problems as swelling during drying and the products sticking together.
When the volume productivity of methods for generating metal coating films is considered, one possibility is to implement electrolytic metal plating conducted with sintered Rxe2x80x94Fexe2x80x94B magnets (cf. Japanese Patent Application Laid-Open No. S60-54406/1985, Japanese Patent Application Laid-Open No. S62-120003/1987), but the surfaces of Rxe2x80x94Fexe2x80x94B bonded magnets are porous and expose a resin portion of low electrical conductivity. As a consequence, plating solution remains, the plating film is not adequately produced on the resin part resulting in pin holes (unplated portions), and rusting occurs.
Thereupon, proposals have been made for selecting plating solution that are harmless even if they penetrate into a porous bonded magnet and remain there (Japanese Patent Application Laid-Open No. H4-276092/1992), and for methods of plating after forming a resin coating on the underlayer (Japanese Patent Application Laid-Open No. H3-11714/1991, Japanese Patent Application Laid-Open No. H4-276095/1992).
It is very difficult, however, to make plating solution completely harmless and these are not solutions that exhibit good film-forming efficiency. Also, the variation in the thickness of the underlayer is a destabilizing factor in plating layers, and to apply an undercoating of sufficient thickness would lead to the contradiction of the plating layer on the surface becoming unnecessary.
Plating solutions of specific compositions have been proposed as a method for implementing nickel plating with good film-forming efficiency on Rxe2x80x94Fexe2x80x94B bonded magnets (Japanese Patent Application Laid-Open No. H4-99192/1992), but here again there is still a danger that such solutions will penetrate into the bonded magnet, remain there, and cause rusting.
In terms of the structural material, on the other hand, the copper strike plating customarily performed prior to nickel plating is either strongly alkaline or strongly acidic, and hence is not suitable for processing Rxe2x80x94Fexe2x80x94B bonded magnets.
In order to impart wear resistance to electronic components, furthermore, and as an anticorrosion treatment for automobile steel panels and the like, practical NiP plating has been developed of a high-temperature acidic solution type, but this is unsuitable for application to Rxe2x80x94Fexe2x80x94B bonded magnets because it causes corrosion in the interior of the magnet.
Thereupon, in the interest of providing Rxe2x80x94Fexe2x80x94B bonded magnets, and a method of manufacture therefore, configured such that plating solution and cleaning fluids, etc., are prevented from penetrating into and remaining in porous Rxe2x80x94Fexe2x80x94B bonded magnets, wherewith a nickel-plated layer or other plating layer can be formed efficiently, and wherewith corrosion resistance and heat resistance can be sharply improved, a method has been proposed wherewith the magnet is subjected to a process for impregnating it with a resin or an inorganic material such as glass to impregnate the pores in the magnet with the resin or inorganic material such as glass, wherewith a surface polishing treatment is then performed such as a barrel-polishing treatment or sandblasting treatment.
Such impregnation and surface polishing treatments are indeed able to modify the surfaces of Rxe2x80x94Fexe2x80x94B bonded magnets while preserving the impregnation effects. However, these are wet polishing treatments and are therefore unsuitable for such easily rusted materials as Rxe2x80x94Fexe2x80x94B bonded magnets due to the corrosion resistance problem. In other words, corrosion resistance deteriorates with rusting developing from the interior so that the plating layer peels away, etc.
A method has also been proposed wherewith the magnet is coated with a mixture of a resin and an electrically conducting powder, an electrically conducting film layer is formed on the surface of the bonded magnet material, and then a surface smoothing treatment is performed (Japanese Patent Application Laid-Open No. H8-186016/1996).
Both of the methods described above are undesirable, however, because various resins are employed to seal the holes in the magnet material, leading unavoidably to the tedious processing steps of resin application (impregnation), curing, and smoothing treatment, and thus, from an industrial perspective, these methods inherently involve the possibility of higher costs.
With a method for coating (impregnating) the magnet material with a resin, it is very difficult to uniformly coat the resin onto the material surface, and, even if barrel-polishing is performed in a subsequent process step, it is very difficult to obtain products exhibiting outstanding dimensional precision. Performing the polishing in a wet process is also problematic in terms of corrosion resistance.
Today, meanwhile, Rxe2x80x94Fexe2x80x94B bonded magnets are being used in more and more applications, and, in applications used in various kinds of electronic equipment installed in automobiles, for example, high corrosion resistance is demanded in Rxe2x80x94Fexe2x80x94B bonded magnets which do not rust in high-temperature high-humidity tests.
When such corrosion resistance is considered, the corrosion-resistant coating layer provided in the magnet surface that is modified by surface-polishing in a dry method so that plating solution and the like are prevented from penetrating must be provided uniformly with even better bonding characteristics.
One object of the present invention is to provide Rxe2x80x94Fexe2x80x94B bonded magnets that exhibit high corrosion resistance and will not rust even in long-duration high-temperature high-humidity tests. Another object is to provide a manufacturing method wherewith various corrosion-resistant coating films can be formed on the Rxe2x80x94Fexe2x80x94B bonded magnets uniformly and with high bonding strength in order to realize high corrosion resistance.
Another object of the present invention is to provide a manufacturing method for highly corrosion-resistant Rxe2x80x94Fexe2x80x94B bonded magnets, comprising optimum industrial process steps for effecting corrosion-resistant coating films with high bonding strength and good dimensional precision on magnet surfaces that prevent plating solution and cleaning fluids, etc., from penetrating into and remaining in porous Rxe2x80x94Fexe2x80x94B bonded magnets.
Intent on resolving such problems, the inventors learned that, by barrel-polishing a porous Rxe2x80x94Fexe2x80x94B bonded magnet in a dry method, using as a medium a mixture of an abrasive material such as abrasive stone formed by sintering inorganic powder of Al2O3, SiC, ZrO, or MgO, or metal balls, and a vegetable medium such as sawdust, fruit rind, or corncobs, or, alternatively, a mixture of an abrasive material noted above and a vegetable medium whose surface has been modified with an inorganic powder as noted above, the surface of the magnet can be smoothed and sealed.
When the mechanism of the barrel polishing noted above was exactingly investigated, it was found that, in dry barrel polishing, it is possible, by means of the oil component in the vegetable medium used, both to bond the grindings of the surface-oxidized layers of the magnetic powder configuring a bonded magnet and the inorganic powder for modifying the surface of the vegetable medium and the powder of the abrasive material to the porous portion of that magnet, thus sealing the magnet, and simultaneously to smooth the surface thereof. From these facts it was learned that it is possible to form a non-electrolytic plating layer with a neutral or alkali solution directly to the surface of the magnet material surface after the dry barrel polishing, and that, by then forming an electrolytic plating layer, a plating film exhibiting significantly superior bonding strength and dimensional precision is obtained. Thus it was learned that a highly corrosion-resistant Rxe2x80x94Fexe2x80x94B bonded magnet exhibiting the outstanding dimensional precision sought can be obtained, and the present invention was perfected.
In other words, the present invention is a method for manufacturing highly corrosion-resistant Rxe2x80x94Fexe2x80x94B bonded magnets wherein, using as a medium a mixture of an abrasive agent and either a vegetable medium or a vegetable medium the surface whereof has been modified with an inorganic powder, the Rxe2x80x94Fexe2x80x94B bonded magnet isbarrel-polished in a dry method, powder of the abrasive agent and grindings of the bonded magnet, or also the inorganic powder, are bonded with the oil component of the vegetable medium to the porous portion of the Rxe2x80x94Fexe2x80x94B bonded magnet, both sealing that magnet and smoothing the surface thereof to modify it, after which a non-electrolytic plating layer is formed directly to the surface of that bonded magnet with a neutral or alkaline solution, and an electrolytic plating layer is then formed.